Circulating fluid hypothermia method

ABSTRACT

A method for performing hypothermia of a selected organ without significant effect on surrounding organs or other tissues. A flexible coaxial catheter is inserted through the vascular system of a patient to place the distal tip of the catheter in an artery feeding the selected organ. A chilled perfluorocarbon fluid is pumped through an insulated inner supply conduit of the catheter to cool a flexible bellows shaped heat transfer element in the distal tip of the catheter. The heat transfer bellows cools the blood flowing through the artery, to cool the selected organ, distal to the tip of the catheter.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of pending U.S. Ser. No.09/052,545, filed Mar. 31, 1998, titled "Circulating Fluid HypothermiaMethod And Apparatus".

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The current invention relates to selective cooling, or hypothermia, ofan organ, such as the brain, by cooling the blood flowing into theorgan. This cooling can protect the tissue from injury caused by anoxiaor trauma.

2. Background Information

Organs of the human body, such as the brain, kidney, and heart, aremaintained at a constant temperature of approximately 37° C. Cooling oforgans below 35° C. is known to provide cellular protection from anoxicdamage caused by a disruption of blood supply, or by trauma. Cooling canalso reduce swelling associated with these injuries.

Hypothermia is currently utilized in medicine and is sometimes performedto protect the brain from injury. Cooling of the brain is generallyaccomplished through whole body cooling to create a condition of totalbody hypothermia in the range of 20° to 30° C. This cooling isaccomplished by immersing the patient in ice, by using cooling blankets,or by cooling the blood flowing externally through a cardiopulmonarybypass machine. U.S. Pat. No. 3,425,419 to Dato and U.S. Pat. No.5,486,208 to Ginsburg disclose catheters for cooling the blood to createtotal body hypothermia.

Total body hypothermia to provide organ protection has a number ofdrawbacks. First, it creates cardiovascular problems, such as cardiacarrhythmias, reduced cardiac output, and increased systemic vascularresistance. These side effects can result in organ damage. These sideeffects are believed to be caused reflexively in response to thereduction in core body temperature. Second, total body hypothermia isdifficult to administer. Immersing a patient in ice water clearly hasits associated problems. Placement on cardiopulmonary bypass requiressurgical intervention and specialists to operate the machine, and it isassociated with a number of complications including bleeding and volumeoverload. Third, the time required to reduce the body temperature andthe organ temperature is prolonged. Minimizing the time between injuryand the onset of cooling has been shown to produce better clinicaloutcomes.

Some physicians have immersed the patient's head in ice to provide braincooling. There are also cooling helmets, or head gear, to perform thesame. This approach suffers from the problems of slow cool down and poortemperature control due to the temperature gradient that must beestablished externally to internally. It has also been shown thatcomplications associated with total body cooling, such as arrhythmia anddecreased cardiac output, can also be caused by cooling of the face andhead only.

Selective organ hypothermia has been studied by Schwartz, et. al.Utilizing baboons, blood was circulated and cooled externally from thebody via the femoral artery and returned to the body through the carotidartery. This study showed that the brain could be selectively cooled totemperatures of 20° C. without reducing the temperature of the entirebody. Subsequently, cardiovascular complications associated with totalbody hypothermia did not occur. However, external circulation of theblood for cooling has not yet been a widely accepted approach for thetreatment of humans. The risks of infection, bleeding, and fluidimbalance are great. Also, at least two arterial vessels must bepunctured and cannulated. Further, percutaneous cannulation of thecarotid artery is very difficult and potentially fatal, due to theassociated arterial wall trauma. Also, this method could not be used tocool organs such as the kidneys, where the renal arteries cannot bedirectly cannulated percutaneously.

Selective organ hypothermia has also been attempted by perfusing theorgan with a cold solution, such as saline or perflourocarbons. This iscommonly done to protect the heart during heart surgery and is referredto as cardioplegia. This procedure has a number of drawbacks, includinglimited time of administration due to excessive volume accumulation,cost and inconvenience of maintaining the perfusate, and lack ofeffectiveness due to temperature dilution from the blood. Temperaturedilution by the blood is a particular problem in high blood flow organssuch as the brain. For cardioplegia, the blood flow to the heart isminimized, and therefore this effect is minimized.

Intravascular, selective organ hypothermia, created by cooling the bloodflowing into the organ, is the ideal method. First, because only thetarget organ is cooled, complications associated with total bodyhypothermia are avoided. Second, because the blood is cooledintravascularly, or in situ, problems associated with externalcirculation of blood are eliminated. Third, only a single puncture andarterial vessel cannulation is required, and it can be performed at aneasily accessible artery such as the femoral, subclavian, or brachial.Fourth, cold perfusate solutions are not required, thus eliminatingproblems with excessive fluid accumulation. This also eliminates thetime, cost, and handling issues associated with providing andmaintaining cold perfusate solution. Fifth, rapid cooling can beachieved. Sixth, precise temperature control is possible.

Previous inventors have disclosed the circulation of cold water orsaline solution through an uninsulated catheter in a major vessel of thebody to produce total body hypothermia. This approach has not beensuccessful at selective organ hypothermia, for reasons demonstratedbelow.

The important factor related to catheter development for selective organhypothermia is the small size of the typical feeding artery, and theneed to prevent a significant reduction in blood flow when the catheteris placed in the artery. A significant reduction in blood flow wouldresult in ischemic organ damage. While the diameter of the major vesselsof the body, such as the vena cava and aorta, are as large as 15 to 20mm., the diameter of the feeding artery of an organ is typically only4.0 to 8.0 mm. Thus, a catheter residing in one of these arteries cannotbe much larger than 2.0 to 3.0 mm. in outside diameter. It is notpractical to construct a selective organ hypothermia catheter of thissmall size using the circulation of cold water or saline solution. Usingthe brain as an example, this point will be illustrated.

The brain typically has a blood flow rate of approximately 500 to 750cc/min. Two carotid arteries feed this blood supply to the brain. Theinternal carotid is a small diameter artery that branches off of thecommon carotid near the angle of the jaw. To cool the brain, it isimportant to place some of the cooling portion of the catheter into theinternal carotid artery, so as to minimize cooling of the face via theexternal carotid, since face cooling can result in complications, asdiscussed above. It would be desirable to cool the blood in this arterydown to 32° C., to achieve the desired cooling of the brain. To cool theblood in this artery by a 5°°C. drop, from 37° C. down to 32° C.,requires between 100 and 150 watts of refrigeration power.

In order to reach the internal carotid artery from a femoral insertionpoint, an overall catheter length of approximately 100 cm. would berequired. To avoid undue blockage of the blood flow, the outsidediameter of the catheter can not exceed approximately 2 mm. Assuming acoaxial construction, this limitation in diameter would dictate aninternal supply tube of about 0.70 mm. diameter, with return flow beingbetween the internal tube and the external tube.

A catheter based on the circulation of fluid operates on the principleof transferring heat from the blood to raise the temperature of thewater. The fluid must warm up to absorb heat and produce cooling. Waterflowing at the rate of 5.0 grams/sec, at an initial temperature of 0° C.and warming up to 5° C., can absorb 100 watts of heat. Thus, the outersurface of the heat transfer element could only be maintained at 5° C.,instead of 0° C. This will require the heat transfer element to have asurface area of approximately 1225 mm². If a catheter of approximately2.0 mm. diameter is assumed, the length of the heat transfer elementwould have to be approximately 20 cm.

In actuality, if circulated through an uninsulated catheter, the wateror saline solution would undoubtedly warm up before it reached the heattransfer element, and provision of 0° C. water at the heat transferelement would be impossible. Circulating a cold liquid through anuninsulated catheter also would cause cooling along the catheter bodyand could result in non-specific or total body hypothermia. Furthermore,to achieve this heat transfer rate, 5 grams/sec of water flow arerequired. To circulate water through a 100 cm. long, 0.70 mm. diametersupply tube at this rate produces a pressure drop of more than 3000 psi.This pressure exceeds the safety levels of many flexible medical gradeplastic catheters. Further, it is doubtful whether a water pump that cangenerate these pressures and flow rates can be placed in an operatingroom.

BRIEF SUMMARY OF THE INVENTION

The selective organ cooling achieved by the present invention isaccomplished by placing a coaxial cooling catheter into the feedingartery of the organ. Cold perfluorocarbon fluid is circulated throughthe catheter. In the catheter, a shaft or body section would carry theperfluorocarbon fluid to a distal flexible heat transfer element wherecooling would occur. A preferred heat transfer element would be bellowsshaped. Cooling of the catheter tip to temperatures above minus 2° C.results in cooling of the blood flowing into the organ located distallyof the catheter tip, and subsequent cooling of the target organ. Forexample, the catheter could be placed into the internal carotid artery,to cool the brain. The size and location of this artery placessignificant demands on the size and flexibility of the catheter.Specifically, the outside diameter of the catheter must be minimized, sothat the catheter can fit into the artery without compromising bloodflow. An appropriate catheter for this application would have a flexiblebody of 70 to 100 cm. in length and 2.0 to 3.0 mm. in outside diameter.

It is important for the catheter to be flexible in order to successfullynavigate the arterial path, and this is especially true of the distalend of the catheter. So, the distal end of the catheter must have aflexible heat transfer element, which is composed of a material whichconducts heat better than the remainder of the catheter. The catheterbody material could be nylon or PBAX, and the heat transfer elementcould be made from a material having higher thermal conductivity, suchas nitinol, nickel, copper, silver, or gold. Ideally, the heat transferelement is formed with a maximized or convoluted surface area, such as abellows. A bellows has a convoluted surface, with fin-like annularfolds, causing the bellows to be very flexible, even though the bellowsis constructed of a metallic material. Further, the convoluted surfaceof the bellows causes it to have a much larger surface area than astraight tube of the same length. Still further, the bellows is axiallycompressible, making it ideal for use on the tip of a catheter whichwill be navigated through the vascular system of a patient. If thebellows abuts the wall of an artery, the bellows will easily compress,thereby eliminating or reducing the trauma to the arterial wall.

Bellows can be formed with known techniques to be seamless andnon-porous, therefore being impermeable to gas. Metallic bellows in thesizes appropriate for use in the present invention are known, which havehelium leak rates less than 10⁻⁶ cc/sec. Impermeability and low leakageare particularly important for use in the present invention, whererefrigerant gas will be circulated through the vascular system of apatient.

Bellows are also mechanically robust, being capable of withstanding morethan 10,000 cycles of axial loading and unloading. Further, metallicbellows are known to tolerate the cryogenic temperatures generated inthe device of the present invention, without loss of performance.Finally, metallic bellows can be made in large quantities, relativelyinexpensively, making them ideal for use in disposable medical device.

Because the catheter body and heat transfer element of the presentinvention may dwell in the vascular system of the patient for extendedperiods, up to 48 hours in some cases, they might be susceptible toblood clot formation if no preventive measures are taken. This isparticularly true of the bellows design, because some blood stasis mayoccur in the annular folds of the bellows, allowing clot forming agentsto cling to the bellows surface and form a thrombus. Therefore,treatment of the catheter body and bellows surfaces to prevent clotformation is desirable. One such treatment is the binding ofantithrombogenic agents, such as heparin, to the surface. Another suchtreatment is the bombardment of the surface with ions, such as nitrogenions, to harden and smooth the surface, thereby preventing clot formingagents from clinging to the surface.

The heat transfer element would require sufficient surface area toabsorb 100 to 150 watts of heat, in the carotid artery example. Thiscould be accomplished with a bellows element of approximately 2 mm.diameter, 13 cm. in length, with a surface temperature of 0° C. Thecooling would be provided by the circulation of a perfluorocarbon fluidthrough an inner supply tube, returning through the annular spacebetween the inner supply tube and the outer tubular catheter body. Theinner tube has insulating means, such as longitudinal channels. Thelongitudinal channels can be filled with a gas, or evacuated. Further,the outer tube of the catheter body can have insulating means, such aslongitudinal channels, which also can be filled with a gas or evacuated.

For example, a perfluorocarbon fluid flowing at a flow rate of betweentwo (2) and three (3) grams/sec could provide between approximately 100and 150 watts of refrigeration power. Utilizing an insulated catheterallows the cooling to be focused at the heat transfer element, therebyeliminating cooling along the catheter body. Utilizing perfluorocarbonfluid also lowers the fluid flow rate requirement, as compared to wateror saline solution, to remove the necessary amount of heat from theblood. This is important because the required small diameter of thecatheter would have higher pressure drops at higher flow rates.

The catheter would be built in a coaxial construction with a 1.25 mm.inner supply tube diameter and a 2.5 mm. outer return tube diameter.This results in tolerable pressure drops of the fluid along the catheterlength, as well as minimizing the catheter size to facilitate carotidplacement. The inner tube would carry the perfluorocarbon fluid to theheat transfer bellows element at the distal end of the catheter body. Ifa bellows surface temperature of 0° C. is maintained, just above thefreezing point of blood, then 940 mm² of surface area in contact withthe blood are required to lower the temperature of the blood by thespecified 5° C. drop. This translates to a 2.0 mm. diameter heattransfer bellows by 13 cm. in length. To generate 0° C. on the bellowssurface, the perfluorocarbon fluid must be supplied at a temperature ofminus 50° C.

It is important to use a perfluorocarbon fluid, for several reasons.First, these compounds have very low viscosities, even at lowtemperatures. Therefore, they can be circulated through small tubing athigh flow rates, with much less pressure drop than water or salinesolution. Second, they can be cooled below 0° C. without freezing,allowing colder fluid to be delivered to the heat transfer element.Since more heat can be transferred to the lower temperature heattransfer element, lower flow rates are required to achieve the samecooling capacity as higher flow rates of water or saline. This furtherdecreases the pressure drop experienced in the tubing. Since somewarming is likely to occur along the catheter body, it is also helpfulto use a fluid which can be cooled to a lower temperature, therebydelivering the desired cooling capacity at the distal tip of thecatheter.

Third, perfluorocarbon fluids have very low surface tension, as comparedto water or saline solution. This is important in applications where theheat transfer element has highly convoluted surface contours to maximizesurface area, such as the bellows heat transfer element. A fluid withlow surface tension will wet the internal surface of such a heattransfer element, resulting in increased heat transfer, whereas a fluidwith a higher surface tension, such as water, which will not completelywet the surface. Fourth, perfluorocarbon fluids are inert and non-toxiccompounds, even having been used as blood substitutes. This is animportant safety concern, in the event of a leak. Finally,perfluorocarbon fluids are chemically compatible with many of theplastics which are used in making catheters. This is important, sincecatheter deterioration could be a serious problem in applications wherethe catheter may remain in the vascular system of the patient,circulating fluid, for long periods of time.

The novel features of this invention, as well as the invention itself,will be best understood from the attached drawings, taken along with thefollowing description, in which similar reference characters refer tosimilar parts, and in which:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic, partially in section, of the apparatus of thepresent invention, showing a first embodiment of the flexible catheter;

FIG. 2 is a perspective view of a second embodiment of the distal tip ofthe catheter;

FIG. 3 is a section view of a third embodiment of the distal tip of thecatheter;

FIG. 4 is a partial section view of a fourth embodiment of the distaltip of the catheter;

FIG. 5 is an elevation view of a fifth embodiment of the distal tip ofthe catheter;

FIG. 6 is an elevation view of the embodiment shown in FIG. 5, aftertransformation to a double helix;

FIG. 7 is an elevation view of the embodiment shown in FIG. 5, aftertransformation to a looped coil;

FIG. 8 is an elevation view of a sixth embodiment of the distal tip ofthe catheter, showing longitudinal fins on the heat transfer element;

FIG. 9 is an end view of the embodiment shown in FIG. 8;

FIG. 10 is an elevation view of a seventh embodiment of the distal tipof the catheter, showing annular fins on the heat transfer element;

FIG. 11 is an end view of the embodiment shown in FIG. 10;

FIG. 12 is a longitudinal section of the distal tip of an eighthembodiment of the catheter of the present invention, showing a bellowsheat transfer element; and

FIG. 13 is a transverse section of one embodiment of an insulated singlewall tube for use in the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, the apparatus includes a flexible coaxial catheterassembly 10, fed by a pumping or circulating unit 12, which can includea pump 13 and a chiller 15, with a freon based refrigerant loop 17. Thecirculating unit 12 has an outlet 14 and an inlet 16. The catheterassembly 10 has an outer flexible catheter body 18, which can be made ofbraided PBAX or other suitable catheter material. The catheter assembly10 also has an inner flexible perfluorocarbon supply conduit 20, whichcan be made of nylon, polyimide, PBAX, or other suitable cathetermaterial. Both the catheter body 18 and the supply conduit 20 should beinsulated, with a preferred means of insulation being discussed in moredetail below.

The lumen 19 of the catheter body 18 serves as the return flow path forthe circulating perfluorocarbon. The catheter body 18 and the supplyconduit 20 must be flexible, to enable passage through the vascularsystem of the patient to the feeding artery of the selected organ. Thelength and diameter of the catheter body 18 and perfluorocarbon supplyconduit 20 are designed for the size and location of the artery in whichthe apparatus will be used. For use in the internal carotid artery toachieve hypothermia of the brain, the catheter body 18 andperfluorocarbon supply conduit 20 will have a length of approximately 70to 100 centimeters. The catheter body 18 for this application will havean outside diameter of approximately 2.5 millimeters and an insidediameter of approximately 2.0 millimeters, and the perfluorocarbonsupply conduit 20 will have an outside diameter of approximately 1.25millimeter and an inside diameter of approximately 1.0 millimeter. Asupply conduit 20 and a return flow path through a catheter body 18 ofthese diameters will have a perfluorocarbon pressure drop ofsignificantly less than the pressure drop that would be exhibited bywater or a saline solution.

The circulating unit outlet 14 is attached in fluid flow communication,by known means, to a proximal end of the perfluorocarbon supply conduit20 disposed coaxially within said catheter body 18. The distal end ofthe perfluorocarbon supply conduit 20 has an outlet adjacent to orwithin a chamber of a flexible heat transfer element such as the hollowflexible tube 24. The tube 24 shown in this embodiment is flexible butessentially straight in its unflexed state. The heat transfer elementmust be flexible, to enable passage through the vascular system of thepatient to the feeding artery of the selected organ. For the internalcarotid application the flexible tube 24 will have a length ofapproximately 15 centimeters, an outside diameter of approximately 1.9millimeters and an inside diameter of approximately 1.5 millimeters. Theheat transfer element also includes a plug 26 in the distal end of theflexible tube 24. The plug 26 can be epoxy potting material, plastic, ora metal such as stainless steel or gold. A tapered transition of epoxypotting material can be provided between the catheter body 18 and theflexible tube 24.

A perfluorocarbon, such as FC-77, made by Dupont, is chilled and pumpedthrough the perfluorocarbon supply conduit 20 into the interior chamberof the heat transfer element, such as the flexible tube 24, therebycooling the heat transfer element 24. Blood in the feeding artery flowsaround the heat transfer element 24, thereby being cooled. The bloodthen continues to flow distally into the selected organ, thereby coolingthe organ. FC-77 is a suitable perfluorocarbon, because it has afreezing point of -110° C., a viscosity of 0.8 centistokes at 25° C.,and a surface tension of 15 dynes per centimeter. Other suitableperfluorocarbons include FC-72 and FC-75, also made by Dupont.

A second embodiment of the heat transfer element is shown in FIG. 2.This embodiment can be constructed of a tubular material such asnitinol, which has a temperature dependent shape memory. The heattransfer element 28 can be originally shaped like the flexible tube 24shown in FIG. 1, at room temperature, but trained to take on the coiledtubular shape shown in FIG. 2 at a lower temperature. This allows easierinsertion of the catheter assembly 10 through the vascular system of thepatient, with the essentially straight but flexible tubular shape,similar to the flexible tube 24. Then, when the heat transfer element isat the desired location in the feeding artery, such as the internalcarotid artery, circulation of chilled perfluorocarbon is commenced. Asthe chilled perfluorocarbon cools the heat transfer element down, theheat transfer element takes on the shape of the heat transfer coil 28shown in FIG. 2. This enhances the heat transfer capacity, whilelimiting the length of the heat transfer element.

A third embodiment of the heat transfer element is shown in FIG. 3. Inthis embodiment, the perfluorocarbon supply conduit 20 has an outlet 30in an interior chamber 32 at the distal end of the heat transferelement. The heat transfer element is a plurality of hollow tubes 34leading from the interior chamber 32 of the heat transfer element to theperfluorocarbon return lumen 19 of the catheter body 18. This embodimentof the heat transfer element 34 can be constructed of a tubular materialsuch as nitinol, which has a temperature dependent shape memory, or someother tubular material having a permanent bias toward a curved shape.The heat transfer element tubes 34 can be essentially straight,originally, at room temperature, but trained to take on the outwardlyflexed "basket" shape shown in FIG. 3 at a lower temperature. Thisallows easier insertion of the catheter assembly 10 through the vascularsystem of the patient, with the essentially straight but flexible tubes.Then, when the heat transfer element 34 is at the desired location inthe feeding artery, such as the internal carotid artery, refrigerantflow is commenced. As the expanding refrigerant cools the heat transferelement 34 down, the heat transfer element takes on the basket shapeshown in FIG. 3. This enhances the heat transfer capacity, whilelimiting the length of the heat transfer element.

A fourth embodiment of the heat transfer element is shown in FIG. 4.This embodiment can be constructed of a material such as nitinol. Theheat transfer element 36 can be originally shaped as a long loopconnecting the distal end of the catheter body 18 to the distal end ofthe perfluorocarbon supply conduit 20, at room temperature, but trainedto take on the coiled tubular shape shown in FIG. 4 at a lowertemperature, with the heat transfer element 36 coiled around theperfluorocarbon supply conduit 20. This allows easier insertion of thecatheter assembly 10 through the vascular system of the patient, withthe essentially straight but flexible tubular loop shape. Then, when theheat transfer element 36 is at the desired location in the feedingartery, such as the internal carotid artery, circulation of chilledperfluorocarbon is commenced. As the chilled perfluorocarbon cools theheat transfer element 36 down, the heat transfer element 36 takes on theshape of the coil shown in FIG. 4. The convoluted surface of this coilenhances the heat transfer capacity, while limiting the length of theheat transfer element 36. FIG. 4 further illustrates that a thermocouple38 can be incorporated into the catheter body 18 for temperature sensingpurposes.

Yet a fifth embodiment of the heat transfer element is shown in FIGS. 5,6, and 7. This embodiment of the heat transfer element can beconstructed of a material such as nitinol. The heat transfer element isoriginally shaped as a long loop 40 extending from the distal ends ofthe catheter body 18 and the perfluorocarbon supply conduit 20, at roomtemperature. The long loop 40 has two sides 42, 44, which aresubstantially straight but flexible at room temperature. The sides 42,44 of the long loop 40 can be trained to take on the double helicalshape shown in FIG. 6 at a lower temperature, with the two sides 42, 44of the heat transfer element 40 coiled around each other. Alternatively,the sides 42, 44 of the long loop 40 can be trained to take on thelooped coil shape shown in FIG. 7 at a lower temperature, with each ofthe two sides 42, 44 of the heat transfer element 40 coiledindependently. Either of these shapes allows easy insertion of thecatheter assembly 10 through the vascular system of the patient, withthe essentially straight but flexible tubular loop shape. Then, when theheat transfer element 40 is at the desired location in the feedingartery, such as the internal carotid artery, circulation of chilledperfluorocarbon is commenced. As the chilled perfluorocarbon cools theheat transfer element 40 down, the heat transfer element 40 takes on thedouble helical shape shown in FIG. 6 or the looped coil shape shown inFIG. 7. Both of these configurations give the heat transfer element 40 aconvoluted surface, thereby enhancing the heat transfer capacity, whilelimiting the length of the heat transfer element 40.

As shown in FIGS. 8 through 11, the tubular heat transfer element 24 canhave external fins 46, 48 attached thereto, such as by welding orbrazing, to give the heat transfer element 24 a convoluted surface,thereby promoting heat transfer. Use of a convoluted surface, such asfins, allows the use of a shorter heat transfer element without reducingthe heat transfer surface area, or increases the heat transfer surfacearea for a given length. In FIGS. 8 and 9, a plurality of longitudinalfins 46 are attached to the heat transfer element 24. The heat transferelement 24 in such an embodiment can have a diameter of approximately1.0 millimeter, while each of the fins 46 can have a width ofapproximately 0.5 millimeter and a thickness of approximately 0.12millimeter. This will give the heat transfer element an overall diameterof approximately 2.0 millimeters, still allowing the catheter to beinserted into the internal carotid artery.

In FIGS. 10 and 11, a plurality of annular fins 48 are attached to theheat transfer element 24. The heat transfer element 24 in such anembodiment can have a diameter of approximately 1.0 millimeter, whileeach of the fins 48 can have a width of approximately 0.5 millimeter anda thickness of approximately 0.12 millimeter. This will give the heattransfer element an overall diameter of approximately 2.0 millimeters,still allowing the catheter to be inserted into the internal carotidartery.

As shown in FIG. 12, the present invention can include a bellows shapedheat transfer element 23 on the distal end of the catheter body 18. Theheat transfer bellows 23 can be constructed of a metal having a highthermal conductivity, such as nitinol, nickel, copper, silver, gold, orsome other suitable material. The catheter body 18 of braided PBAX canhave a distal section 21 of non-braided PBAX. The heat transfer bellows23 comprises a tubular throat 25 fitted within the distal section 21 ofthe catheter. A plurality of annular folds 27 extend from the distal endof the throat 25. An end cap 29 is formed at the distal end of theannular folds 27.

The distal section 21 of non-braided PBAX can be melted or shrunk ontothe bellows throat 25. A shrink fit tube 31 of a suitable plastic can beformed onto the distal section 21 of the catheter and the bellows throat25, to form a smooth transition. To prevent thrombosis, anantithrombogenic agent such as heparin can be bonded to the outersurface 33 of the bellows 23, particularly on the annular folds 27.Alternatively, the outer surface 33 of the bellows 23 can be nitrided orsubjected to a similar treatment to create a smooth surface whichretards thrombosis.

The inner perfluorocarbon supply conduit 20 should be insulated toprevent warming of the cold supply perfluorocarbon by the warmer returnperfluorocarbon in the outer lumen 19. In one embodiment, a single wallperfluorocarbon supply conduit 20 could be constructed with a ring ofparallel longitudinal lumens in the wall of the conduit 20, surroundingthe cold perfluorocarbon flow path. These parallel longitudinal lumenscould be evacuated, actively or passively, or filled with insulatingmaterial. Similarly, the outer lumen 19 of the catheter body 18, whichcarries the return perfluorocarbon flow, could be insulated, to reducethe warming of the return flow by the blood surrounding the catheterbody 18. FIG. 13 is a transverse section of an insulated single walltube which could be used for either the insulated catheter body 18 orthe supply conduit 20, or both. The catheter body 18 or supply conduit20 has a single wall 50, with an inner lumen 52 for flow of theperfluorocarbon. A plurality of longitudinal insulating lumens 54 arearranged surrounding, and parallel to, the inner lumen 52. Eachinsulating lumen 54 can be evacuated during manufacture of thecirculating catheter assembly 10. If evacuated during manufacture, eachof the insulating lumens 54 could be sealed at its proximal end,creating a constant, passive vacuum in each of the insulating lumens 54.Alternatively, the insulating lumens 54 could be evacuated during use ofthe circulating catheter assembly 10, such as by the use of a vacuumpump or syringe (not shown). Conversely, the insulating lumens 54 couldbe filled with an insulating material. As mentioned above, thisinsulated single wall design could be used for the supply conduit 20, orthe catheter body 18, or both.

While the particular invention as herein shown and disclosed in detailis fully capable of obtaining the objects and providing the advantageshereinbefore stated, it is to be understood that this disclosure ismerely illustrative of the presently preferred embodiments of theinvention and that no limitations are intended other than as describedin the appended claims.

I claim:
 1. A method for selective organ hypothermia, said methodcomprising:providing a circulating fluid apparatus having a chiller anda flexible coaxial catheter, said catheter having an insulated innerlumen and a hollow flexible heat transfer element adjacent its distaltip; inserting said catheter through the vascular system of a patient toplace said heat transfer element in a feeding artery of a selectedorgan; supplying chilled perfluorocarbon fluid to said insulated innerlumen of said coaxial catheter; cooling the interior of said heattransfer element with said chilled perfluorocarbon fluid; cooling bloodflowing in said feeding artery with said heat transfer element, toenable said cooled blood to flow distally into said selected organ andcool said organ; and returning said perfluorocarbon fluid to saidchiller.
 2. A method for selective organ hypothermia, said methodcomprising:providing a coaxial catheter, said catheter having aninsulated inner lumen and a metallic heat transfer element; introducingsaid coaxial catheter into the vascular system of a patient to placesaid metallic heat transfer element in a feeding artery of an organ ofthe patient; cooling said metallic heat transfer element by circulatinga refrigerant through said insulated inner lumen of said coaxialcatheter; cooling blood in said feeding artery by contact with saidcooled metallic heat transfer element; and cooling said organ by flow ofsaid cooled blood through said feeding artery.
 3. A method for selectivebrain hypothermia, comprising:providing a flexible coaxial catheter,said flexible catheter having an insulated inner lumen and a flexiblemetallic heat transfer element; introducing said flexible coaxialcatheter into the vascular system of a patient to place said flexiblemetallic heat transfer element in the carotid artery of the patient;cooling said flexible metallic heat transfer element by circulating arefrigerant through said insulated inner lumen of said flexible coaxialcatheter; cooling blood in said carotid artery by contact with saidcooled flexible metallic heat transfer element; and cooling the brain ofthe patient by flow of said cooled blood through said carotid artery. 4.A method for selective hypothermia of the heart of a patient,comprising:providing a flexible coaxial catheter, said flexible coaxialcatheter having an insulated inner lumen and a flexible metallic heattransfer element; introducing said flexible coaxial catheter into thevascular system of a patient to place said flexible metallic heattransfer element in a feeding artery of the heart of the patient;cooling said flexible metallic heat transfer element by circulating arefrigerant through said insulated inner lumen of said flexible coaxialcatheter; cooling blood in said feeding artery by contact with saidcooled flexible metallic heat transfer element; and cooling the heart ofthe patient by flow of said cooled blood through said feeding artery. 5.A method for selective organ hypothermia, said methodcomprising:providing a circulating fluid apparatus having a chiller anda flexible coaxial catheter, said catheter having an inner lumen and ahollow flexible heat transfer element adjacent its distal tip; insertingsaid catheter through the vascular system of a patient to place saidheat transfer element in a feeding artery of a selected organ; supplyingchilled perfluorocarbon fluid to said inner lumen of said coaxialcatheter; cooling the interior of said heat transfer element with saidchilled perfluorocarbon fluid; cooling blood flowing in said feedingartery with said heat transfer element, to enable said cooled blood toflow distally into said selected organ and cool said organ; andreturning said perfluorocarbon fluid to said chiller.
 6. A method fororgan hypothermia, said method comprising:providing a coaxial catheter,said catheter having an inner lumen and a metallic heat transferelement; introducing said coaxial catheter into the vascular system of apatient to place said metallic heat transfer element in an artery of anorgan of the patient; cooling said metallic heat transfer element bycirculating a refrigerant through said inner lumen of said coaxialcatheter; cooling blood in said artery by contact with said cooledmetallic heat transfer element; and cooling said organ by flow of saidcooled blood through said artery.
 7. A method for selective brainhypothermia, comprising:providing a flexible coaxial catheter, saidflexible catheter having an inner lumen and a flexible heat transferelement; introducing said flexible coaxial catheter into the vascularsystem of a patient to place said flexible heat transfer element in thecarotid artery of the patient; cooling said flexible heat transferelement by circulating a refrigerant through said inner lumen of saidflexible coaxial catheter; cooling blood in said carotid artery bycontact with said cooled flexible heat transfer element; and cooling thebrain of the patient by flow of said cooled blood through said carotidartery.
 8. A method for selective hypothermia of the heart of a patient,comprising:providing a flexible coaxial catheter, said flexible coaxialcatheter having an inner lumen and a flexible heat transfer element;introducing said flexible coaxial catheter into the vascular system of apatient to place said flexible heat transfer element in a feeding arteryof the heart of the patient; cooling said flexible heat transfer elementby circulating a refrigerant through said inner lumen of said flexiblecoaxial catheter; cooling blood in said feeding artery by contact withsaid cooled flexible heat transfer element; and cooling the heart of thepatient by flow of said cooled blood through said feeding artery.